In optical communications, it is desirable to use a communication wavelength in the wavelength range of approximately 1.5 micrometers (.mu.m). This is because 1.5 .mu.m is the lowest loss wavelength of conventional single-mode glass fibers. As the number of desired channels and demand for high video quality increases, the corresponding demands on an optical transmission source also increase. Among these is the demand for a low-noise, single-frequency 1.5 .mu.m laser source. Fiber laser sources have been created which use an erbium (Er) doped optical fiber as a gain medium. However, such fiber lasers typically suffer from a tradeoff between gain and power, and mode discrimination.
It is commonly understood that, within a fiber laser, longitudinal mode spacing is inversely proportional to the length of the laser cavity. That is, for a longer resonator cavity, the frequency spacing between adjacent resonance frequencies is smaller. As a result, more longitudinal modes are within the (necessarily finite) width of the gain spectrum, and mode discrimination is poor. Even if the effective gain spectrum of the fiber laser is narrowed through the use of narrow band reflectors, such as Bragg gratings written directly into the core of the fiber, many modes may exist with that effective gain bandwidth. For the designer of a single-frequency laser, this causes several difficulties. Firstly, the wide spectrum emitted from the laser interferes with other laser sources transmitted at different wavelengths in wavelength division multiplexed systems. Secondly, dispersion in the wide spectrum results in pulse spreading. Finally, mode competition between longitudinal modes causes amplitude instability.
By making the laser cavity shorter, the mode spacing is increased and, if the cavity is short enough, the gain spectrum may be limited to a single resonance frequency. However, the length of the gain medium (i.e. the doped fiber surrounding the cavity) also affects the optical gain of the laser. With a relatively short length of fiber, the gain of the laser is low since there is less distance across which the pump energy may be absorbed within the fiber.
It would be desirable to have an optical source that has an output power significantly higher than previously available, and which has the mode discrimination benefits of a short fiber gain medium.